Lithium-ion batteries (LIBs) are important components of portable electronic devices and are rapidly being introduced into the electric and hybrid vehicle markets. Future applications may include storage for renewable power sources and load leveling for grids.
In an effort to increase the energy density of lithium-ion batteries, high voltage cathodes that insert lithium at ˜5V have been proposed. However, at these high potentials one must consider electrolyte solutions that are stable at high potentials. In addition, electrolyte oxidation can occur not only on the surface of the active material, but also on the surface of carbon, which is a common conductive additive in positive electrodes in lithium ion batteries. In an effort to increase the electrochemical window of lithium ion battery, we have examined carbons of different surface morphology and measured their reactivity towards electrolyte oxidation.
Carbon modifications are reported in the scientific literature and are the subject of numerous patents. Most of these, however, deal with the modification of graphitic materials for use as active components in anodes. A recent publication of Winter et. al. provides a good overview in the introduction about recent developments in the field of graphite modification (Journal of Power Sources, Volume 200, 15 Feb. 2012, Pages 83-91).
Looking specifically at carbon black materials, the number of efforts taken and improvements observed decreases drastically. To date, carbon black has not been thoroughly investigated in all of its possibilities due in part to the complex structure of the different carbon blacks and the use of it “only” as an additive. Some modifications examples include the use of acids or oxidizing agents. (Materials Chemistry and Physics, Volume 76, Issue 1, 28 July 2002, Pages 1-6). Therefore, there is no evidence that the herein presented modifications have been investigated so far.
Lithium-ion battery composite electrodes commonly incorporate 1-10% wt. of carbon black additives in their formulation. The promising new positive electrode materials for high-energy Li-ion batteries e.g., LiMnPO4, LiNi0.5Mn1.5O4, x Li[Mn1/2Ni1/2]O2.y LiCoO2.z Li[Li1/3Mn2/3]O2 (x+y+z=1), which operate at relatively high potentials require new electrode materials definitions for stable performance in organic carbonate electrolytes.
Interfacial processes at Li-ion positive composite electrodes have been studied quite extensively. Organic carbonate-based electrolytes undergo oxidation at the surface of positive electrodes at potentials exceeding 4.2 V vs. Li/Li+, which often leads to degradation of the electrode active and passive components. These processes result in gradual electrolyte degradation, surface film formation and gas evolution during cell operation, which affect electrochemical performance and lifetime of Li-ion cells. Most of these studies tend to focus on the electrochemical properties of the electrode active material itself, often ignoring the fact that carbon black (CB) conductive additives constitute 80-98% of the composite electrode surface area. Investigations of interfacial properties of CB additives in Li-ion negative electrodes are manifold. However, studies of electrochemical properties of carbons at potentials of lithium-ion positive electrodes are limited and tend to focus mainly at anion intercalation into graphite.
Organic carbonate electrolytes begin to undergo oxidative decomposition at carbonaceous surfaces at potentials above 4.2V. Electrolyte decomposition products adversely affect the electrochemical impedance of the electrode and they are responsible for lithium inventory shift and premature failure of Li-ion cells.